1. Field of the Invention
The invention relates to arrangements for path protection for optical communications networks. More specifically, the invention relates to arrangements for path protection that involve intelligent monitoring and selection from among plural incoming signals that may have taken at least partially geographically diverse paths to arrive at their destination, yet in a way that reduces hardware costs by avoiding a need for duplicative transponders or other optical elements.
2. Related Art
It is known in the art to provide “1+1” path protection to signals in optical communications networks. Briefly, “1+1” path protection involves embedding the same information in optical signals sent via geographically diverse pathways. In this arrangement, if one pathway is broken by malfunction, fiber cut or the like, then the information may be switched to reach its destination through the other pathway(s).
Alternative “1+1” schemes are shown in FIGS. 1A and 1B. A first site 141 is connected to a second site 142 via plural optical pathways such as first pathway 31 and second pathway 32. Sites 141 and 142, which may be hundreds or thousands of kilometers apart, may be considered to constitute nodes in an optical communications network. First and second pathways 31, 32 may each include amplifiers, intervening sites, and other elements that are not specifically shown. Pathways 31, 32 in “1+1” schemes may traverse geographically different paths so that any malfunction, fiber cut, or the like, along one pathway is less likely to affect the other pathway than if the pathways were not geographically diverse.
FIGS. 1A, 1B illustrate an internal architecture of site 141, with the understanding that site 142 may have an identical or similar architecture.
FIGS. 1A, 1B each illustrate a wavelength selective switch (WSS) 10 configured to route optical signals between and among three bidirectional optical pathways W, N, and E. (Of course, W, N, and E are arbitrary designation of the pathways, and the pathways are not required to extend in respective westward, northward, or eastward directions.)
Wavelengths incoming on optical pathway W from an optical amplifier A and passive coupler (or splitter) 1 are sent to Nx1 switches 104 and 106 for the N and E pathways, respectively. Likewise, wavelengths incoming on optical pathway N from an optical amplifier C and a passive coupler (or splitter) 3 are sent to Nx1 switches 102 and 106 for the W and E pathways, respectively. Similarly, wavelengths incoming on optical pathway E from an optical amplifier e and passive coupler (or splitter) 5 are sent to Nx1 switches 102 and 104 for the W and N pathways, respectively.
For output from the WSS, the output of Nx1 switch 102 is provided to output pathway W by driving an optical amplifier B. Likewise, the output of Nx1 switch 104 is provided to output pathway N by driving an optical amplifier D. Similarly, the output of Nx1 switch 106 is provided to output pathway E by driving an optical amplifier F.
The FIG. 1A, 1B architecture reflects an approach to optical adding and dropping called “broadcast-and-select.” In conventional broadcast-and-select, only one of switches 102, 106 transmits the outbound signal provided by coupler 3. The other switch of 102, 106 does not transmit the signal, so that the signal only goes out on one of pathways W or E, not on both pathways.
Site 141 also has a local add-drop terminal (ADT) 20 that communicates with WSS 10 via bidirectional optical pathway N. Terminal 20 is controlled by an element 30 that communicates with elements (not shown) in higher layers of a communications protocol stack. Element 30, which may be a router or protocol multiplexing device, also provides network layer control and data signals to the lower level (data link layer) WSS 10. In the present context, router 30 may be considered to constitute, or at least interface with, a client.
Within terminal 20 is a multiplex/demultiplex (MUX/DEMUX) element 24 that communicates on bidirectional pathway N with WSS 10. MUX/DEMUX element 24 also communicates with each of plural (for example, two) bi-directional transponders 26A, 26B (collectively called element 26). Each bidirectional transponder communicates with router 30.
Each bi-directional transponder includes, in series leading from MUX/DEMUX element 24: an optical-line element 26OL, an electronics element 26E, and an optical-tributary element 26OT, all of which may be of conventional design. The optical-tributary element is the client-facing portion of the transponders and constitutes the short-reach optics connecting the terminal to the client. Electronics element 26E performs the regeneration, re-amplification and re-timing function (“3R”) and other transponder functionality. Finally, optical-line element 26OL is a tunable long-reach laser and receiver for generating the outbound interoffice signal, and for receiving such signals from other sites.
In FIG. 1A, it is the client that provides the 1+1 functionality. This functionality may be accomplished, for example, by providing a second laser and receiver on the line card of router 30, together with the electronics needed to monitor the health of the incoming signals and switch between them as needed. The entire line card might need to be duplicated. However, because much of the functionality of the backbone router is likely to be located on the line card, this arrangement raises significant cost problems.
In contrast to the FIG. 1A arrangement, FIG. 1B shows an approach in which there need only be a single laser/receiver on the line card of router 30. However, a separate “1+1” subsystem 32 is required, to split the outbound signal and receive the two incoming signals and switches between them as needed. Subsystem 32 performs quality analysis on incoming signals. Subsystem 32 could be in a discrete box separate from the client equipment 30 and integrated optical cross-connect (IOXC) terminal 20, or it could be physically within the IOXC terminal 20 with cabling to the two IOXC terminal transponders 26A, 26B.
Unfortunately, the arrangements in FIGS. 1A and 1B have significant shortcomings, especially in terms of cost. In particular, both arrangements require plural transponders, plus additional hardware either within router 20 (FIG. 1A) or as an additional subsystem 32 (FIG. 1B). Thus, there is a need in the art to provide increased communication reliability through path protection without incurring the increase costs of systems such as that shown in FIGS. 1A and 1B.
More generally, it is known to provide a “protection” path as an alternate to a “working” path. For example, U.S. Patent Application Publication No. 2004/0179472 (Khalilzadeh et al.) discloses an arrangement in which a traffic signal on a working path is monitored, and upon a drop node's detection of a failure in the traffic signal, the network forwards the traffic signal on a protection path. However, such arrangements do not inherently provide that traffic is simultaneously carried on the two paths, and the setting up the protection path traffic delays data transmission.
Other arrangements involve switching the same data between the working and protection paths. For example, U.S. Patent Application Publication No. 2002/0176131 (Walters et al.) discloses protection switching involving first and second optical cards connected to the working and protection paths, as well as an “inter card communication channel” that allows a one of the paths to be connected to an opposite optical card.
Because of the requirement for a second card, this arrangement appears to have shortcomings similar to the arrangements in FIGS. 1A and 1B.
Still other arrangements involve distributed control of protection switching. For example, U.S. Pat. No. 6,144,633 (Ikeda et al.) discloses an arrangement in which each node in a network stores information concerning the connection status of all other nodes, so as to optimize line switching in the event of a failure. However, this arrangement is not only complex and costly in terms of required hardware and control structure, but fails to involve simultaneous transmission of the same signals that permits simple and reliable restoration of service in the event of one path's failure.
Thus, undesirably, arrangements like those in FIGS. 1A and 1B require a doubling of the terminal optics and their attendant high costs. Moreover, none of the documents discussed above provide path protection simply, or at low cost. Accordingly, there is a need in the art for practical, low cost path protection in optical communications networks.